The use of reinforced composite materials to produce structural components is now widespread, particularly in applications where their desirable characteristics for being lightweight, strong, tough, thermally resistant, self-supporting and their adaptability to being formed and shaped are sought. Such components are used, for example, in the aeronautical, aerospace, and satellite industries, as well as for recreational uses such as in racing boats and countless other applications. Typically components consist of reinforcement materials embedded in matrix materials. The reinforcement component may be made from materials such as glass, carbon, ceramic, aramid, polyethylene, and/or other materials which exhibit desired physical, thermal, chemical and/or other properties, chief among which is great strength against stress failure. Through the use of such reinforcement materials, which ultimately become a constituent element of the completed component, the desired characteristics of the reinforcement materials such as very high strength, are imparted to the completed composite component. The constituent reinforcement materials may typically be woven, knitted or otherwise oriented into desired configurations and shapes for reinforcement preforms. Usually, particular attention is paid to ensure the optimum utilization of the properties for which these constituent reinforcing materials have been selected. Generally, such reinforcement preforms are combined with matrix material to form desired finished components or produce working stock for the ultimate production of finished components.
After a desired reinforcement preform has been constructed, matrix material may be introduced and combined with the preform, so that the reinforcement preform becomes encased in the matrix material such that the matrix material fills the interstitial areas between the constituent elements of the reinforcement preform. The matrix material may be any of a wide variety of materials, such as epoxy, polyester, vinyl-ester, ceramic, carbon and/or other materials, which also exhibit desired physical, thermal, chemical and/or other properties. The materials chosen for use as the matrix may or may not be the same as that of the reinforcement preform and may or may not have comparable physical, chemical thermal or other properties. Typically, however, they will not be of the same materials or have comparable physical, chemical, thermal or other properties, as the reinforcement preform, since a usual objective sought in using composites in the first place is to achieve a combination of characteristics in the finished product that is not attainable through the use of one constituent material alone. When combined, the reinforcement preform and the matrix material may then be cured and stabilized in the same operation by thermosetting or other known methods, and then subjected to other operations toward producing the desired component. It is significant to note that after being so cured, the then solidified masses of the matrix material are normally very strongly adhered to the reinforcing material (e.g., the reinforcement preform). As a result, stress on the finished component, particularly via its matrix material acting as an adhesive between fibers, may be effectively transferred to and borne by the constituent material of the reinforcing reinforcement preform.
Frequently, it is desirable to produce components in configurations other than simple geometric shapes such as plates, sheets, rectangular or square solids, etc. Of particular interest is the configuration consisting of a flat substrate with integral sidewalls in two directions. The integral sidewalls can be used as conventional stiffeners in stiffened skin applications, wherein the substrate acts as the skin. In each of these applications, it is important to make each juncture between the constituent components, i.e. the stiffener and the base platform or panel portion, as strong as possible. Given the desired very high strength of the reinforcement preform constituents per se, weakness of the juncture becomes, effectively, a “weak link” in a structural “chain.”
Various methods have been used in the past for joining composite components or reinforcement preforms to produce a reinforced complex structure or stiffened skin panels. It has been proposed to use individual layers of tackified fabric or prepreg to form the structure of the substrate with sidewalls. When tackified fabric is used, the resulting laminated preform can thereafter be resin transfer molded; if prepreg is used to form the final shape, the resulting preform can be vacuum bagged and cured. However, application of loads, in particular through thickness loads, to the laminated preform can result in delamination of the preform or separation of the sidewalls from the flat substrate.
The use of metal bolts or rivets at the interface of such components has also been used but is unacceptable because such additions at least partially destroy and weaken the integrity of composite structures themselves, add weight, and introduce differences in the coefficient of thermal expansion between such elements and the surrounding material.
Other approaches to solving this problem have been based on the concept of introducing high strength fibers across the joint area through the use of such methods as stitching one of the components to the other and relying upon the stitching thread to introduce such strengthening fibers into and across the juncture site. One such approach is shown in U.S. Pat. No. 4,331,495 and its method divisional counterpart, U.S. Pat. No. 4,256,790. These patents disclose junctures having been made between a first and second composite panel made from adhesively bonded fiber plies. The first panel is bifurcated at one end to form two divergent, co-planar panel contact surfaces in the prior art manner, that have been joined to the second panel by stitches of uncured flexible composite thread through both panels. The panels and thread are then “co-cured”: i.e., cured simultaneously. However, this process requires the preform to be constructed in multiple steps, as well as requires the introduction of a third yarn or fiber into the preform.
An example of an intersecting configuration is set forth in U.S. Pat. No. 6,103,337, the disclosure of which is incorporated herein by reference. This reference discloses a means for joining a reinforcement preform with a preform panel to form a three-dimensional reinforcement preform. The two individual preforms are joined to each other at the junction by means of reinforcing fibers in the form of threads or yarns. Once the two preforms are joined or stitched together, matrix material is introduced to the preforms. However, this process requires that the preforms be individually woven or constructed and subsequently stitched together in a separate step. The preforms are not continuously or integrally woven together. Furthermore, an additional yarn or fiber is needed to connect the preforms.
Another method to improve upon junction strength is set forth in U.S. Pat. No. 5,429,853. However, this method is similar to previously described methods because separately constructed distinct elements are joined together by the stitching of a third yarn or fiber between the two.
While the prior art has sought to improve upon the structural integrity of the reinforced composite and has achieved some success, there exists a desire to improve thereon and to address the problem through an approach different from the use of adhesives or mechanical coupling of the separate panel and stiffener elements. In this regard, one approach might be by creating a woven three-dimensional structure on specialized machines. A three-dimensional fabric generally consists of fibers extending along a direction perpendicular to the other fibers, which is along the X, Y and Z axial directions. However, the expense involved is considerable and rarely is it desirable to have a weaving machine directed to creating a single type of structure.
Another approach is to weave a two-dimensional structure and fold it into shape so that the panel is integrally stiffened, i.e. yarns are continuously interwoven between the planar base or panel portion and the stiffener. However, this typically results in distortion of the preform when the preform is folded. The distortion occurs because the lengths of fiber as-woven are different than what they should be when the preform is folded. This causes dimples and ripples in areas where the as-woven fiber lengths are too short, and buckles in the areas where fiber lengths are too long. These distortions cause undesirable surface anomalies and reduce the strength and stiffness of the component. While this may be relieved by cutting and darting, such procedures are undesirable since they are labor intensive or otherwise may compromise the integrity of the preform.
U.S. Pat. No. 6,446,675, the disclosure of which is incorporated herein by reference, solves the problem with distortion that occurs upon folding a two-dimensional woven preform by adjusting the lengths of the fibers during weaving such that some fibers are too short in some areas and others too long in other areas. Upon folding the preform, the lengths of the fibers are equalized, providing for a smooth transition at the fold. However, this woven preform is only capable of providing reinforcement or stiffening in one direction, which is parallel to the warp fiber direction.
Another approach for constructing stiffened panels is set forth in U.S. Pat. No. 6,019,138 which discloses a method for making stiffened panels with reinforcing stiffeners in both the warp and fill directions. As disclosed, this method achieves reinforcement in two directions through over weaving, or simply weaving high spots into the panel portion of the preform. Using this method will limit the height of the stiffener that can be achieved. Further, this method requires that the preform be woven using three yarns. The third yarn, which binds the stiffener to the panel portion of the preform, is only periodically woven between the other two yarns. Therefore, the stiffener is not completely integrally woven with the panel portion which results in a joint that is weaker than a fully integrally woven joint.
Accordingly, a need exists for an integrally woven preform that provides sidewalls in two or more directions that can be woven in one process using a conventional loom without any special modifications.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.